Intersymbol interference is a form of distortion that has long been an obstacle to the error-free reception of digital symbols. Indeed, with the burgeoning growth of high-speed digital communications, intersymbol interference is perhaps the major impediment to the accurate reception of digital information. This distortion phenomenon results from the fact that a pulse propagating through a band-limited channel expands in the time domain. Accordingly, each received symbol, ideally equal to a particular transmitted symbol, is now a combination of the transmitted symbols.
Intersymbol interference is characterized as being linear or nonlinear. In linear intersymbol interference, each received symbol is a weighted linear sum of an associated transmitted symbol--which the received symbol ideally represents in the absence of distortion--along with other transmitted symbols which precede and succeed the associated symbol in time. The weighting coefficient for each transmitted symbol, while varying in time, is independent of the sequence of transmitted symbols. In nonlinear intersymbol interference, while each received symbol is also a function of an associated transmitted symbol along with preceding and succeeding transmitted symbols, the weighting coefficient for each symbol is a function of the transmitted symbol sequence. Hence, each received symbol represents a linear combination of products of the associated transmitted symbol and preceding and succeeding transmitted symbols and/or the complex conjugate of such symbols.
Because the weighting coefficients in linear intersymbol interference are independent of the transmitted symbol sequence, this form of intersymbol interference is easier to analyze and a number of techniques have been quite successful in compensating for such distortion. These techniques include linear feedforward equalization and decision feedback equalization. In accordance with the former technique, each received symbol is added to a weighted linear sum of past and future symbols prior to a decision being made as to the value of the transmitted symbol. In accordance with the latter technique, a weighted linear sum of past decisions is added to each received symbol, again prior to a decision being made as to the value of the transmitted symbol. See, for example, U.S. Pat. No. 3,974,449 issued to D. D. Falconer on Aug. 10, 1976.
A number of techniques are also known for the compensation of nonlinear intersymbol interference. See, for example, U.S. Pat. No. 3,600,681 to T. Arbuckle issued Aug. 17, 1971 and U.S. Pat. No. 4,181,888 and No. 4,213,095 issued to D. D. Falconer on Jan. 1, 1980 and July 15, 1980, respectively. These cited techniques, while somewhat successful, have either not been fully effective in compensating for nonlinear intersymbol interference and/or require circuitry whose complexity grows rapidly with the order of nonlinearity in the transmission channel. This latter shortcoming can make the known techniques ill-suited from a cost standpoint for many systems applications. Therefore, a technique for effectively compensating for both linear and nonlinear intersymbol interference without the need for complex hardware arrangements would be desirable.